mid-15c., "full of pride," from Latin fastidiosus "disdainful, squeamish, exacting," from fastidium "loathing, squeamishness; dislike, aversion; excessive nicety," which is of uncertain origin; perhaps from *fastu-taidiom, a compound of fastus "contempt, arrogance, pride," and taedium "aversion, disgust." Fastus is possibly from PIE *bhars- (1) "projection, bristle, point," on the notion of "prickliness" (Watkins) or "a semantic shift from 'top' to 'haughtiness' which is conceivable, but the u-stem is not attested independently" [de Vaan], who adds that "fastidium would be a tautology." Early use in English was both in passive and active senses. Meaning "squeamish, over-nice" in English emerged 1610s. Related: Fastidiously; fastidiousness.
late 14c., variant (with -ish) of squoymous "disdainful, fastidious" (early 14c.), from Anglo-French escoymous, which is of unknown origin. Related: Squeamishly; squeamishness.
He was somdel squaymous
Of fartyng, and of speche daungerous
[Chaucer, "Miller's Tale," c. 1386]
1530s "gloomy, of a sour temper, sullen and austere," from Latin morosus "morose, peevish, hypercritical, fastidious," from mos (genitive moris) "habit, custom" (see moral (adj.)). In English, manners by itself means "(good) manners," but here the implication in Latin is "(bad) manners."